The present invention relates generally to the formation of adherent coatings on substrates and, more particularly, to the formation of adherent coatings on substrates receptive to carbon, nitrogen or boron ion implantation. This invention was made with government support under Contract No. W-7405-ENG-36 awarded by the U.S. Department of Energy to The Regents of the University of California. The government has certain rights in the invention.
Processing by plasma assisted techniques is increasingly used in various areas of production. One of the most promising uses is in the deposition of coatings. Such applications are expanding in response to requirements for lower processing temperatures, reduced environmental impact, and enhanced performance. To meet the needs of the civil and military industrial complexes, new and novel synthesis approaches must be scaleable, produce conformal coatings, be environmentally benign, and be economical to implement.
Many industries routinely utilize coatings and other surface treatments to reduce wear and improve corrosion resistance. A critical issue in traditional coatings is delamination or separation at interfaces due to poor chemical bonding and/or from excessive compressive stress that often accompanies thick coatings. Additionally, since molten metal and wet chemical baths are usually employed in conventional methods, presently used coating techniques generate dangerous vapors, utilize toxic materials, and pose environmental hazards related to the disposal of generated wastes. As a result, a new, innovative, and highly versatile implementation approach is required.
Potentially environmentally benign coating technologies (e.g., PVD, CVD, plasma sprays, and line-of-sight ion implantation) have been available for decades. In spite of their perceived flexibility and promise, traditional line-of-sight surface modification techniques are generally considered to be too expensive, too slow, and too complicated for mass production applications. For example, the deposit of diamond-like carbon coatings using line-of-sight implantation is described in U.S. Pat. No. 5,391,407 for xe2x80x9cProcess For Forming Protective Diamond-Like Carbon Coatings On Metallic Surfaces,xe2x80x9d which issued to Geoffrey Dearnaley on Feb. 21, 1995, and in U.S. Pat. No. 5,393,572 for xe2x80x9cIon Beam Assisted Method Of Producing A Diamond Like Carbon Coating,xe2x80x9d which issued to Geoffrey Dearnaley on Feb. 28, 1995.
Plasma Source Ion Implantation (PSII) is discussed by John R. Conrad in U.S. Pat. No. 4,764,394 for xe2x80x9cMethod And Apparatus For Plasma Source Ion Implantation,xe2x80x9d which issued on Aug. 16, 1988. Therein the implantation of ions into surfaces of three-dimensional targets is achieved by forming an ionized plasma about the target within an enclosing chamber and applying a pulse of high voltage between the target and the conductive walls of the chamber, whereby ions from the plasma are driven into the target object surfaces simultaneously from all sides. Plasma Source Ion Implantation has overcome many of the disadvantages of the conventional technologies, but in its present state, is limited to but a few gas sources. However, PSII technology has been successfully applied to improve the tribological properties of automotive parts in U.S. Pat. No. 5,458,927 for xe2x80x9cProcess For The Formation Of Wear- And Scuff- Resistant Carbon Coatings,xe2x80x9d which issued to Gerald W. Malaczynski et al. on Oct. 17, 1995. Therein, an adherent diamond-like carbon coating is formed on an aluminum substrate. However, there is no discussion in Malaczynski et al. concerning relieving the stress between the coating and the substrate which results from the coating process.
It is well known that plasma-generated coatings are intrinsically stressed. See, e.g., xe2x80x9cIntrinsic Stress Scaling Law For Polycrystalline Thin Films Prepared By Ion Beam Sputtering,xe2x80x9d by H. Windischmann, J. Appl. Phys. 62, 1800 (1987), and xe2x80x9cIntrinsic Stress In Sputtered Thin Films,xe2x80x9d by H. Windischmann, J. Vac. Sci. Technol. A 9, 2431 (1991). Such stress is expected to be in part responsible for poor film adhesion to substrates. However, in xe2x80x9cIon Beam Induced Modifications In DC Sputtered TiN/B-C-N Multilayers,xe2x80x9d by S. Fayeulle et al., Nucl. Instr. and Meth. in Phys. Res. B 127/128, 198 (1997), and in xe2x80x9cThermal And Ion Irradiation Stability Of Direct Current Sputtered TiN/B-C-N Multilayers,xe2x80x9d by S. Fayeulle et al., Appl. Phys. Lett. 70, 1098 (1997), ion beam irradiation of deposited surfaces using Ar ions has been found to relax the highly compressive stress initially present in multilayered, sputtered thin films of TiN/B-C-N to a small tensile stress. This should greatly improve the film adherence properties.
Boron carbide and boron nitride thin films have been deposited on surfaces using plasma techniques. See, e.g., xe2x80x9cThe Structural Homogeneity Of Boron Carbide Thin Films Fabricated Using Plasma-Enhanced Chemical Vapor Deposition From B5H9+CH4,xe2x80x9d by Sunwoo Lee et al., J. Appl. Phys. 74, 6919 (1993), and xe2x80x9cBoron Nitride Thin Film Deposition Using Electron Cyclotron Resonance Microwave Plasmas,xe2x80x9d by S. M. Gorbatkin et al., J. Vac. Sci. Technol. A 11, 1863 (1993), respectively.
Accordingly, it is an object of the present invention to overcome two fundamental limitations to the widespread use of environmentally friendly coating technologies by: (1) reducing the time and expense of treating complex shapes and large areas, and (2) extending the capabilities of deposition technologies to provide novel coatings with enhanced adherence and superior performance.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.
To achieve the foregoing and other objects, and in accordance with the purposes of the present invention, as embodied and broadly described herein, the method for forming an adherent coating on a substrate of this invention includes: applying a first, negative-pulsed bias voltage to the substrate; immersing the biased substrate in a plasma containing atoms which strongly bond to the coating, such that ions of the binding atoms are injected into the surface of the substrate, forming thereby a compositionally graded surface thereon; subsequently applying a second, negative-pulsed bias voltage to the substrate; immersing the substrate having the compositionally graded surface in a plasma containing gaseous precursor species which will form the desired coating on the surface thereof; applying a third pulsed, negative bias voltage to the coated substrate; and immersing the coated substrate in a plasma containing inert gas ions such that the stress experienced by the coating is reduced, whereby the coating strongly adheres to the surface of said substrate.
It is preferred that the coating is boron carbide and that the gaseous precursor species which generates the boron carbide coating on the surface of the substrate surface includes acetylene and diborane.
Preferably, the binding atoms injected into the surface of said substrate, forming thereby a compositionally graded surface thereon, are selected from the group consisting of carbon and boron.
It is also preferred that the first, negative-pulse bias voltage is between 10 kV and 100 kV, that the second, negative-pulse voltage is between 50 V and 10 kV, and that the third, negative-pulse bias voltage is between 10 kV and 100 kV.
Preferably also, the process steps are repeated, perhaps using different coating precursor species until a coating having the desired thickness and composition is achieved.
Benefits and advantages of the present invention include the deposition of a substantial variety of coatings having greater adherence than those generated by other techniques on numerous substrates. Additionally, the coating thickness and density can be controlled.